For the transmission of digital data, multi-channel data transmission based on Orthogonal Frequency Division Multiplex (OFDM), also known as Discrete Multitone (DMT) modulation, is a known flexible modulation scheme. OFDM spreads the data to be transmitted over a large number of sub-carriers or sub-channels which are included in a transmission band and separated from each other by a well-defined frequency spacing. The latter can ensure orthogonality of the sub-carriers and prevent crosstalk or inter-carrier interference between sub-carriers (i.e., the demodulator for one sub-carrier is not affected by the modulation of the other sub-carriers even though there is no explicit filtering and their spectra overlap). The individual OFDM modulation symbols on each of the carriers represent a number of bits that depends on the choice of the QAM alphabet (i.e., the arrangement of data or constellation points in the quadrature amplitude plain). For instance, 2 bit/symbol for Quadrature Phase Shift Keying (QPSK), or 4 bit/symbol for 16-QAM (Quadrature Amplitude Modulation) is known. The complex processes of modulating and demodulating thousands of carriers simultaneously are comparable to Discrete Fourier Transform operations, for which efficient Fourier transform algorithms exist.
A suitable OFDM modem architecture includes an encoder to multiplex, synchronize and encode the data to be transferred, as well as a modulator to form a discrete multitone signal. The encoder translates incoming bit streams into in-phase and quadrature components for each of a multiplicity of sub-channels (i.e., the encoder outputs a number of sub-symbol sequences that are equal to the number of sub-channels available to the system). A line monitor at the receiver checks the line quality of the sub-channels (e.g. by repeatedly determining the noise-level, gain and phase-shift individually for each of the sub-channels during operation). The background noise power of the totality of sub-channels in the transmission band as well as the bit error rate (BER) and/or the signal-to-noise ratio (SNR) of each individual sub-channel are then used to determine the channel capacity of the sub-channels (i.e., the information density or bit transmission rate that each sub-channel can support). An optimization signal code construction procedure selects an appropriate QAM alphabet or bit allocation scheme that results in a data rate that approximates the sub-channel capacity by considering conditions of limited signal power and maximum bit error rate.
OFDM is, for example, suited for Power Line Communication (PLC). Power line data channels at high or medium voltage are affected by interferers, because the cable types that are used for the transmission of electric power are unshielded and therefore vulnerable for electromagnetic ingress. A known noise scenario on power line channels resulting there from comprises so-called narrowband interferers (i.e., signals with a small bandwidth originating, for example, from radio transmissions and presenting a spectral amplitude rising up to 40 dB above a background noise level deprived of any contribution from narrowband interferers). Likewise, known analogue television signals essentially behave like narrow-band interferers to OFDM. Thus, the power line channel does not present an additive white Gaussian noise (AWGN) environment, but in the frequency range from some hundred kilohertz up to 20 MHz is mostly dominated by narrow-band interference caused by ingress of broadcast stations with a received level generally varying with daytime, and impulsive noise from switching power supplies or other transient phenomena.
The international patent application WO 97/40609 is concerned with the reduction of radio-frequency (RF) interference from narrow frequency amateur radio bands between 1 MHz and 12 MHz in a wide-band multi-carrier transmission system. A “drop” portion of a standard unshielded twisted-pair ADSL subscriber line covering the last 30 m or less up to a remote unit is found to be able to both receive and emit RF signals. In a restricted band which contains the sub-channels prone to interference, no sub-carriers are used for data transmission. In addition, a dummy tone may be used to suppress transmitted power from sidelobe transmissions within that band.
The patent application EP-A 1 137 194 proposes to determine individual SNRs for the sub-channels in a OFDM system and, based there upon, to reallocate data transmission rate and/or signal power to the sub-channels. According to the U.S. Pat. No. 6,456,653 the overall SNR in OFDM systems is estimated during regular operation by determining a noise power of inactive sub-carriers and a signal plus noise power of active sub-carriers, and subtracting the former from the latter to obtain a signal power.
The disclosures of all documents mentioned herein are hereby incorporated by reference in their entireties.